ENG-4461: Move task log from RejoinProducer to the base class.
ENG-4461: Move clearing of rejoin overflow dir from Iv2RejoinCoordinator to the
base class.

Also renamed the overflow directory from rejoin_overflow to join_overflow.
Add more comment to the JoinProducer.
ENG-4461: Make two methods in JoinProducer publicly visible for tests.
ENG-4461: Add task log to the JoinProducer.
Minor code cleanup in JoinProducer.
ENG-4461: Move TestJoinProducer to pro, since it requires pro code.
Merge pull request #438 from VoltDB/ENG-4461

ENG-4461: Log and replay fragment tasks at the joining SPI